1. Field of the Invention
The present invention relates to a zoom lens unit, an imaging apparatus and a portable information terminal device including the zoom lens unit.
2. Description of the Related Art
In recent years, demands from users regarding functions of imaging apparatuses (for example, digital cameras) have increased. Especially, requirements of high-quality picture performance and compactness for digital cameras need to be improved.
Therefore, both high performance and compactness have naturally been required for a zoom lens unit used as a photographing lens.
From the viewpoint of compactness of a zoom lens unit, firstly it is necessary to shorten a total length of the zoom lens unit (a distance from a lens surface nearest to an object side to an image surface) when using it. In addition, it is important to shorten a thickness of each lens group to suppress a total length of the zoom lens unit when the lens unit is collapsed.
From the viewpoint of high performance of a zoom lens unit, at least a resolution corresponding to an image pickup device having 10 million pixels over all a zoom area is necessary for the zoom lens unit. In addition, preferably a magnification ratio as great as possible is desired. In fact, a zoom lens unit with a focus length of about 28-200 mm (about 7.1 times) in terms of a 35-mm film camera is considered to be capable of responding to almost all general photographing conditions.
In addition, many users require a photographing lens with wide-angle capability, and it is desirable that a half-field angle of a zoom lens unit at a wide-angle end be 38 degrees or more. The half-field angle of 38 degrees corresponds to a focal length of 28 mm in terms of a 35-mm film camera (so-called Leica version).
Furthermore, a macro photography function is one of the demands of the user for the zoom lens unit. The macro photography function is a function which is capable of focusing on a subject located in a closer range than a range of subject distance which is capable of being focused on in usual photographing, and capable of photographing, and it is necessary to obtain a higher image magnification (a wide magnification capability).
Many kinds of zoom lens units for a digital camera are possible. As a kind of zoom lens unit which is suitable for compactness and high magnification, the following zoom lens unit is known from JP 2008-26837 A and JP 2001-059938 A, which includes, in order from an object side, a first lens group having a positive refracting power; a second lens group having a negative refracting power; a third lens group having a positive refracting power; a fourth lens group having a positive refracting power, and when changing magnification from a wide-angle end to a telephoto end, an interval between the first lens group and the second lens group increases, an interval between the second lens group and the third lens group decreases, and an interval between the third lens group and the fourth lens group increases.
Zoom lens units described in JP 2008-26837 A and JP 2001-059938 A, in concrete embodiments, have achieved compactness, high performance, wide field angle and high magnification, and met the demands of the users.
Recently, the macro photography function in a state where a focus length is comparatively long (so-called tele macro function) is a demand for a digital camera by users. This is because, with the tele macro function, by enabling the macro photographing in a state where the focal length is long (a field angle is narrow), even with a same image magnification, there are advantages such as distortion of a subject image by perspective can be suppressed, a working distance can be secured, and a degree of freedom of lighting or the like is increased.
JP 2001-059938 A discloses a zoom lens unit which has four lens groups with positive/negative/positive/positive refracting power, and has a macro photography function, and even though the macro photography function is sufficient, there is still room for improvement in the tele macro photography function.